The foaming of polycarbonates is known, preferred blowing agents being those which give off an inert gas, such as nitrogen or carbon dioxide (see, for example, DE-AS 1,031,507 and DE-OS 2,434,085).
It is also known that oxalic acid can be used as the blowing agent (see U.S. Pat. No. 3,277,029).
Surprisingly, polycarbonates themselves can be used to foam polycarbonates providing CO.sub.2 is released through controlled, partial degradation of the polycarbonate and used to foam the remaining polycarbonate.
Controlled degradation of the polycarbonate is obtained by the addition of transesterification catalysts in combination with aromatic polycarboxylic acids and/or H.sub.2 O. Where H.sub.2 O is used without aromatic polycarboxylic acids, aliphatic hydroxycarboxylic acids and/or aliphatic alcohols may be combined with the addition of the H.sub.2 O.
It is known that polycarbonates can be degraded with release of carbon dioxide under the influence of acids (see DE-AS 1,260,479), even in combination with catalysts (see EP 0,127,842 and U.S. Pat. No. 4,338,422).
However, it is also known that polycarbonates can be stabilized against the effect of moisture and against irradiation by the addition of acids (see U.S. Pat. No. 3,951,903 and EP 0,353,776).
It is also known that satisfactory processing of polycarbonates is only possible in the dry state (see BayerKunststoffe, 2nd Edition, 1959, page 177) and that moisture leads to molecular weight degradation (Plastics Technology, 10, 1964, pages 32-36) or to the formation of blisters (GB-PS 841,652).
It is also known, however, that the addition of H.sub.2 O to polycarbonate followed by vacuum extrusion results in clear polycarbonate material (see U.S. Pat. No. 3,567,813 and EP-0,300,485).
Mixtures of polycarbonates, a particular graft polymer and a polymeric acidifying agent are disclosed in DE-OS 3,325,702. The molding compounds obtainable from the mixtures have improved stability and impact strength.
The addition of fatty acids as mold release agents for polycarbonates is also known (see U.S. Pat. No. 4,409,351 and U.S. Pat. No. 4,408,000). In this case, however, degradation of the polycarbonate also occurs (see DE-OS 3,704,666. Comparison Example 2, and Jp-Sho 47-41092).
EP-A-0,158,212 describes the use of monoesters and diesters of citric acid for the production of foamed plastics. Polycarbonates are among the plastics mentioned.